Green’s Dictionary of Slang

seriously adv.

1. an all-purpose intensifier, e.g. seriously rich.

[UK]N. Lucas Autobiog. of a Thief 224: Do you know, Lucas, the Yard has been seriously after you for several months!
[UK]Barr & York Sloane Ranger Hbk 159: serious, seriously adj & adv. The Sloane way of exaggeration. ‘He’s seriously rich.’.
[US]C. Stroud Close Pursuit (1988) 214: How about you and me get seriously pissed.
[US]G. Tate ‘The GOP Throws a Mammy-Jammy’ in Flyboy in the Buttermilk (1992) 104: Koko Taylor [...] sporting this antique and seriously fly white velour hat wrapped with a silken band.
[UK]H. Mantel Beyond Black 69: Recarro seats. Full spec. Seriously speedy.
[US]M. Lacher On the Bro’d 16: The trucks were seriously big.

2. an expression of agreement.

[US]Eble Campus Sl. Mar. 7: seriously – expression of agreement: X:‘This weather is horrible.’ Y: ‘Seriously.’.